This invention relates to a carburetor for internal combustion engines and is of the type which includes a fuel metering device having a fuel outlet opening situated in the axis of the intake manifold upstream of an arbitrarily operated butterfly throttle valve. The carburetor further has a choke device which is situated upstream of the fuel outlet opening and which serves for the pressure-dependent setting of the air flow passage section of the intake manifold. The choke device includes two components that are generally quadrangular in shape and that are disposed in the flow passage of the intake manifold. Each component is pivotal about an axis that extends generally along an edge of the component. These edges of the one and the other component are parallel to and remote from one another. The components execute pivotal movements simultaneously in mutually opposite directions as a function of the prevailing pressure conditions in the intake manifold and define together -- by means of parts oriented towards one another -- the flow passage section for the intake air. The carburetor according to the invention finds application particularly, but not exclusively, in internal combustion engines associated with motor vehicles.
A carburetor of the above-outlined type as disclosed, for example, in German Laid-Open Application (Offenlegungsschrift) No. 2,201,253 has, in very general terms, the advantage that it ensures a better mixture preparation in all load ranges of the internal combustion engine. In the known carburetor, the device for the pressure-dependent setting of the air flow passage section includes two components which are pivotally supported in the intake manifold and which have such a configuration that during opposed simultaneous pivotal motions, they roll on each other in the central zone of the inner cross-sectional area of the intake manifold and together define a circular flow passage section of variable magnitude. This arrangement has the advantage that for any setting of the device the maximum flow velocity of the air passing through the flow passage section is located in the zone of the fuel outlet of the fuel metering device. It is, however, a disadvantage of the above-outlined known structure that between the outer edges of the two components, on the one hand, and the wall of the intake manifold, on the other hand, there appear relatively wide clearances through which -- particularly in case the device is substantially closed -- there is generated a significant secondary flow situated externally of the zone of the fuel outlet. It is apparent that the air in such secondary flow does not appreciably contact the fuel emitted through the outlet opening of the fuel metering device. Further, the known device has, due to the design of the components as three-dimensional members and due to the arrangement of their pivotal axis in the flow path of the intake air, a disadvantageously large flow resistance.